1. Field of the Invention
The present invention relates to an image forming apparatus adopting a cleanerless process, and particularly to a technique to prevent the deterioration of picture quality due to photoreceptor filming or color mixture.
2. Description of the Related Art
Hitherto, there is known a color image forming apparatus in which development is performed by plural developing units to respectively supply toners of different colors. In the image forming apparatus of the related art as stated above, there is a case where the toner transferred on an intermediate transfer belt or a transfer member is reversely transferred to a photoconductive side in a downstream side transfer unit. In an apparatus in which a cleaner to remove the toner on the photosensitive surface is not provided, the reversely transferred toner enters the developing unit as it is, and a problem of so-called color mixture arises. The color mixture of toner as stated above causes a change in hue of an image to be printed, and causes a reduction in color reproducibility.
In order to suppress the occurrence of the reverse transfer, there is disclosed a technique in which a corona charger is provided between image forming units to prevent the reverse transfer (see, for example, JP-6-75484), a technique in which forming is performed so that a front surface contact width becomes smaller than a back surface contact width and color mixture due to the reverse transfer is prevented (see, for example, JP-A-2000-315023), or a technique in which transfer efficiency is improved, durability is improved by containing a filler, and an intermediate transfer body of a three-layer structure is provided in which a resistance difference of 100 times or more is provided between the respective layers (see, for example, JP-A-11-338266).
However, in the related art, there is a case where the occurrence of photoreceptor damage (flaw, hole, etc.) can not be suppressed according to the material of a surface layer, and filming due to the photoreceptor damage occurs.
Besides, like the technique as disclosed in JP-A-11-338266, in the case where the resistance difference of 100 times or more is provided between the respective layers of the intermediate transfer body of the three-layer structure, there is a fear that the transfer performance of the intermediate transfer body is reduced according to the value of the resistance value of the layer having the highest resistance value, and from the viewpoint that the transfer performance is kept while the reverse transfer is prevented, it is hard to say that the related art discloses the optimum relation of the resistance values of the respective layers of the intermediate transfer body.